Structure of an Oligodeoxynucleotide Containing a Butadiene Oxide-Derived N1 Beta-Hydroxyalkyl Deoxyinosine Adduct in the Human <i>N-ras</i> Codon 61 Sequence<sup>†</sup>

The solution structure of the N1-(1-hydroxy-3-buten-2(<i>S</i>)-yl)-2‘-deoxyinosine adduct arising from the alkylation of adenine N1 by butadiene epoxide (BDO), followed by deamination to deoxyinosine, was determined, in the oligodeoxynucleotide d(CGGA<u>CXA</u>GAAG)·d(CTTCTCGTCCG). This oligodeoxynucleotide contained the BDO adduct at the second position of codon 61 of the human <i>N-ras</i> protooncogene, and was named the <i>ras61</i> <i>S</i>-N1−BDO-(61,2) adduct. <sup>1</sup>H NMR revealed a weak C<sup>5</sup> H1‘ to X<sup>6</sup> H8 NOE, followed by an intense X<sup>6</sup> H8 to X<sup>6</sup> H1‘ NOE. Simultaneously, the X<sup>6</sup> H8 to X<sup>6</sup> H3‘ NOE was weak. The resonance arising from the T<sup>17</sup> imino proton was not observed. <sup>1</sup>H NOEs between the butadiene moiety and the DNA positioned the adduct in the major groove. Structural refinement based upon a total of 364 NOE-derived distance restraints yielded a structure in which the modified deoxyinosine was in the high syn conformation about the glycosyl bond, and T<sup>17</sup>, the complementary nucleotide, was stacked into the helix, but not hydrogen bonded with the adducted inosine. The refined structure provided a plausible hypothesis as to why this N1 deoxyinosine adduct strongly coded for the incorporation of dCTP during trans lesion DNA replication, both in <i>Escherichia coli</i> [Rodriguez, D. A., Kowalczyk, A., Ward, J. B. J., Harris, C. M., Harris, T. M., and Lloyd, R. S. (2001) <i>Environ. Mol. Mutagen. 38</i>, 292−296], and in mammalian cells [Kanuri, M., Nechev, L. N., Tamura, P. J., Harris, C. M., Harris, T. M., and Lloyd, R. S. (2002) <i>Chem. Res. Toxicol. 15</i>, 1572−1580]. Rotation of the N1 deoxyinosine adduct into the high syn conformation may facilitate incorporation of dCTP via Hoogsteen-type templating with deoxyinosine, thus generating A-to-G mutations.